Fuel injectors are used to direct fuel pulses into a combustion chamber. Normally in a fuel supply system, fuel pumps deliver and pressurize fuel from a storage tank to fuel injectors or to an accumulator that connects to fuel injectors. Inside a fuel injector, a nozzle assembly including a nozzle valve (nozzle needle valve) is used to control fuel flow through nozzle orifices. At “off” position, the nozzle valve blocks fuel flow. When the nozzle valve moves to “on” position, fuel is presses through the orifices. The overall fueling rate is determined by the injector “on” time.
Fuel injectors in different applications have different requirements for fuel pressure. For example, in CI (Compression Ignition) engines (diesel engines), high fuel pressure is needed for better fuel atomization and spray penetration, while in SI (Spark Ignition) engines, fuel pressure is lower due to the fuel/air pre-mixing nature. Even in some applications for diesel engines, for example, fuel dosing (used with a DOC in increasing exhaust air temperature) in regenerating DPFs (Diesel Particulate Filters), low pressure fuel systems (<20 bar) are used. When the nozzle valve opens (injector at “on” status), fuel flow rate is a function of fuel pressure for a given injector. Accordingly, in fuel flow control, the injector “on” time is a function of fuel pressure inside the injector.
To decrease emission, increase combustion efficiency, and improve combustion control performance, fuel flow needs to be controlled accurately. However, due to limitations of sensor accuracy, sensor response time, system complexity, and cost, it is hard to implement a feedback loop in fuel flow control. As a result, accurate nozzle valve timing control and steady fuel pressure inside the injector are required.
Another important factor that affects fuel flow control accuracy is injector deterioration or aging. Normally fuel injectors (especially diesel fuel injectors) work in an environment with high temperature and high concentration of particulate matter. Consequently, carbon (and other particles) could build up on the nozzle surface, partially blocking nozzle orifices and deteriorating fueling control performance, resulting in poor system performance and more emission. The injector deterioration is more significant in applications with short operation time. For example, in aftertreatment systems using DPFs, high exhaust temperature is generated by burning fuel in a DOC or combustion chamber (burner), and a fuel injector is used in controlling fueling rate. Since the regeneration is only needed when too much soot accumulates in the filter, the interval between regenerations could be hours or days. In this application, there is long period of time for carbon to build up on injector surface without disturbing from fueling flow. Injector deterioration changes fuel spray pattern and causes large error between actual fueling rate and fueling command. These effects result in changes in system gain (ratio of exhaust temperature increase to fueling rate), and temperature control performance deteriorates therewith.
If the nozzle orifice change is measurable, the value of changed orifice size can be used for compensating fueling control: when orifices become smaller due to deterioration, we can increase injector “on” time to compensate fueling rate change. However, due to limitations of sensor technology and OBD (On-board Diagnostics) technology, it is hard to accurately and reliably measure orifice change in-situ using a compact and economic device.